[Shiny]

I'm pretty sure an inverted flare fitting has straight threads. It appears your flare seat is damaged and the threads are stripped, correct?

You are proposing a tapered male pipe fitting, correct? Since your female flare fitting is useless as-is, is there enough meat to go a size larger with an NPT tap on the radiator side? If you can get NPT threads cut, maybe you can get by. I don't think cutting straight female threads or chasing the existing ones, then trying to use a male NPT fitting will help, but that's just my guess. Definitely better odds if you use a tapered NPT tap in the buggered fitting. And you could try teflon tape if you go this route.